1. Field of the Invention
The present invention relates to a planographic printing plate precursor. More specifically, the invention relates to an infrared-laser-applicable planographic printing plate precursor for a so-called CTP (Computer To Plate), from which a printing plate can be directly formed based on digital signals from a computer or the like.
2. Description of the Related Art
The development of lasers for planographic printing in recent years has been remarkable. In particular, high-power, small-sized solid lasers and semiconductor lasers that emit near-infrared and infrared rays have become easily obtainable. These lasers are very useful as exposure light sources when forming printing plates directly from digital data of computers or the like.
Materials which can be used for positive type planographic printing plate precursors applicable for infrared lasers include, as essential components, a binder resin soluble in an aqueous alkaline solution (hereinafter referred to where appropriate as an “alkali-soluble resin”), and an infra red dye which absorbs light to generate heat. When an image is formed in a positive type planographic printing plate precursor, the infra red dye interacts with the binder resin in its unexposed portions (image portions) so as to function as a dissolution inhibitor which can substantially reduce the solubility of the binder resin. On the other hand, in its exposed portions (non-image portions), interaction of the infra red dye with the binder resin is weakened by the heat generated. Consequently, an exposed portion can turn into a state in which it can be dissolved in an alkaline developer, so that an image is formed thereon and a planographic printing plate is produced.
However, insofar as such infrared-laser-applicable positive planographic printing plate precursor materials are concerned, differences in the degree of resistance against dissolution in a developer between unexposed portions (image portions) and exposed portions (non-image portions) therein, that is, differences in development latitude have not yet been sufficient under various conditions of use. Thus, problems have occurred insofar that, with changes in conditions of use of materials, materials have tended to be either excessively developed or inadequately developed.
Further, when using an infrared-laser-applicable positive type planographic printing plate precursor, if the surface state of the unexposed portions of the plate precursor is slightly changed by human finger touching the surface or some other action, the affected unexposed portions (image portions) are dissolved by development to generate marks like scars. As a result, the plate precursor has problems in that the printing resistance thereof deteriorates and the ink-acceptability thereof worsens.
Such problems stem from fundamental differences in plate-making mechanisms between infrared-laser-applicable positive type planographic printing plate precursor materials and positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violet rays.
Specifically, positive type planographic printing plate precursor materials from which printing plates are made up by exposure to ultra violer rays each include, as essential components, a binder resin soluble in an aqueous alkaline solution and an onium salt, or a quinonediazide compound. This onium salt or quinonediazide compound not only interacts with the binder resin in unexposed portions (image portions) to function as a dissolution inhibitor, but in exposed portions (non-image portions) it is also decomposed by light and generates an acid to function as a dissolution promoter. In this way, the onium salt, or the quinonediazide compound, performs dual functions.
On the other hand, in infrared-laser-applicable positive type planographic printing plate precursor materials, the infra red dye functions only as a dissolution inhibitor of unexposed portions (image portions), and does not promote the dissolution of exposed portions (non-image portions). Therefore, in order to make distinctive the difference in solubility between the unexposed portion and the exposed portion in a positive planographic printing plate material for infrared laser, it is inevitable that a material which already has a high solubility in an alkali developing solution is used as the binder resin. It is therefore the case that the state of the plate material before developed becomes unstable.
Various proposals have been offered to solve the above problems. For example, a method has been proposed in which the distribution of an infrared absorbing agent is localized in the layer to improve the discrimination of an image (see, for example, the publication of Japanese Patent Application Laid-Open (JP-A) No. 2001-281856). Although there is improved discrimination by this method, the problem concerning scratch resistance on the surface of the recording layer has yet to be solved.
Also, a planographic printing plate precursor has been proposed which is provided with a recording layer, comprising a lower layer containing a sulfonamide type acryl resin, and an upper layer, which contains a water-insoluble and alkali-soluble resin and a photo-thermo converting agent, which is improved in solubility in an aqueous alkali solution by exposure to light (see, for example, the publication of JP-A No 11-218914). This type of planographic printing plate precursor produces the effect that, because the lower layer which is highly alkali-soluble is exposed when the recording layer is removed on an exposed portion, undesired residual film and the like are removed smoothly by an alkali developing agent. The lower layer also functions as an insulating layer, so that thermal diffusion to the support is efficiently suppressed. In the planographic printing plate precursor of this type, a method has been proposed in which a polymer is blended in the lower layer to provide chemical resistance (see, for example, a leaflet of International Publication (WO) No. 01/46318).
However, in order to form the multilayer structure, it is essential to select, as the resins used in both layers, those which differ in characteristics from each other, giving rise to the problem that the interaction between these resins may be reduced. Also, because the developing characteristics of the lower layer are so good, there is a possibility that an undesired dissolution phenomenon occurs at both end portions of the lower layer during developing, which adversely affects printing durability and image reproducibility. Therefore, there is ample room to make good use of the merits of a multilayer structure.